Method and a device for bonding a metal fairing to the leading edge of an airfoil

ABSTRACT

A method and a device ( 1 ) for bonding a metal fairing ( 6 ) to protect a leading edge of an aircraft airfoil ( 7 ), the metal fairing ( 6 ) is designed to be bonded against the airfoil ( 7 ) with the help of an adhesive ( 9 ). The bonding device ( 1 ) is remarkable in that it includes a rigid yoke ( 2 ) having inserted therein induction heater elements ( 30 ) for heating the metal fairing ( 6 ).

The present invention relates to a method and to a device for bonding ametal fairing to the leading edge of an aircraft airfoil, e.g. arotorcraft rotor blade.

BACKGROUND OF THE INVENTION

The leading edge of a rotorcraft rotor blade is protected againstimpacts, e.g. by a U-section metal fairing bonded to the structure ofthe blade, or indeed by a fairing made from composite materials astaught in document EP 0 529 917 A, for example. The metal fairing thusprotects the leading edge of the blade and also protects portions of thesuction and pressure sides of the blade that are directly adjacent tothe leading edge.

When the metal fairing is damaged by impacts, or even by an erosionphenomenon, it is appropriate to replace it with a new metal fairing soas to ensure that the blade will continue to be properly protected.

Furthermore, it can be necessary to remove the leading edge in order torepair the structure of the blade, particularly if the blade is made ofcomposite material and surface layers of the structure are delaminating.Once the repair has been made, the leading edge needs to be put backinto place.

A device is known that enables the metal fairing for the leading edge ofa blade to be stuck back into position by using an adhesive that isplaced between the metal fairing and the structure of the blade.

That device makes use of heater tooling that delivers the energy neededto polymerize an adhesive so as to secure the metal fairing to thestructure of the blade, the adhesive being placed between the metalfairing and the structure of the blade. The heater tooling isconstituted by a mat that is placed on the outside surface of the metalfairing.

The mat contains a heater resistance that is embedded in a layer ofsilicone. On being powered electrically, the heater resistance heats up,thereby enabling the metal fairing to be bonded to the structure of theblade. The heater resistance then heats the metal fairing by conduction,which fairing in turn transmits heat to the adhesive, thereby causing itto polymerize.

That device operates properly. Nevertheless, it presents a fewdrawbacks.

The first drawback is a consequence of using an electrical resistance.All of the elements surrounded by the heating mat are then heated by themat, i.e. simultaneously the metal fairing, the adhesive, and thestructure. That leads to energy consumption that is not necessarilyoptimized.

Similarly, a second drawback lies in an edge effect. A large amount ofheat is lost by conduction from the edges of the device, which meansthat the device operates non-uniformly. Certain zones of the assemblycomprising the metal fairing and the blade structure are thus heated toa smaller extent than others, thereby ending up with over-consumption ofenergy in order to compensate for the losses.

A third drawback is a consequence of the temperature rise times of theheater resistance, with rates of rise being very slow, of the order of1.5 degrees Celsius per minute (° C./min). The duration of the cycle forheating the metal fairing can then easily amount to eight hours, whichis penalizing industrially speaking from an economic point of view.

Furthermore, the airfoil is subjected to large temperature stresses overa long period due to the slow rises in temperature, and that can degradethe airfoil.

Finally, fabricating the mat is very lengthy, and therefore expensiveand incompatible with urgent industrial demand, as can happenunfortunately when a manufacturer in possession of a heater mat that isin poor condition needs to repair a blade.

Document WO 01/30116 discloses a bonding device that includes inductionheater means for melting an adhesive. Nevertheless, it would appear tobe difficult to apply that bonding device to the metal fairing of theleading edge of an airfoil.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is thus to provide a device forbonding a metal fairing uniformly and quickly to a leading edge, whileavoiding the limitations of the above-mentioned devices.

According to the invention, a bonding device for bonding a protectivemetal fairing on the leading edge of an aircraft airfoil, which metalfairing is to be bonded against the airfoil with the help of anadhesive, is remarkable in that it comprises a rigid yoke in whichinduction heater means are inserted for heating the metal fairing forbonding to the airfoil.

Consequently, the induction heater means heats the metal fairing, whichin turn heats the adhesive by conduction so as to cause it topolymerize. More precisely, the induction heater means create a varyingmagnetic field made up of a plurality of magnetic field lines thatinduce electromotive forces (emfs) within the metal material of themetal fairing. These electromotive forces then induce currents known aseddy currents that heat the material of the metal fairing by the Jouleeffect.

There are no losses due to edge effects, nor are there any due toelements being heated that are not involved with bonding the metalfairing on the leading edge, since only metal elements are heated byinduction.

The induction heater means thus heat up the metal fairing very quickly,in particular because of the lack of heat losses. In addition, thismethod of heating is much faster than heating by conduction, e.g. in themanner implemented in the prior art.

Furthermore, the temperature of the adhesive is uniform insofar as it ispossible for the magnetic field lines generated by the induction heatermeans also to be distributed uniformly without difficulty. The lack oflosses due to edge effects guarantees that the temperature of theadhesive is more uniform, where such uniformity is of very greatimportance when bonding the leading edge fairing.

By way of example, it is then not necessary to heat the metal fairingfor longer than the design time, since the temperatures to which it isheated are under full control.

Finally, since the heater device heats only the metal fairing, and theadhesive is heated by conduction from the fairing, the device does notrun the risk of damaging the structure of the airfoil by heating itexcessively.

Advantageously, the induction heater means are arranged between firstand second primary layers of a thermo-expandable material, e.g. of thesilicone type, the second primary layer matching the shape of the metalfairing while the device is in use.

The bonding device is then provided in succession with a rigid andnon-deformable yoke, the first primary layer, the induction heatermeans, and then the second primary layer.

In addition, metal fairings for leading edges are conventionally ofU-section. Thus, the first and second primary layers are also ofU-section. The second primary layer then comes into contact with theoutside surface of the metal fairing, while the inside surface of themetal fairing is in contact with the adhesive for bonding it to theairfoil.

Furthermore, the first and second primary layers are made of athermo-expandable material. Thus, when the induction heater means heatsup the metal fairing, the fairing delivers a certain amount of heat tothe second primary layer by conduction.

Because of its consistency, the second primary layer then begins toexpand. Since the yoke is rigid, it cannot deform under the effect ofthe second primary layer expanding. The second primary layer thereforeexerts pressure on the metal fairing.

This phenomenon is of particular importance since it contributes tosecuring the metal fairing properly to the airfoil.

Under the action of the induction heater means, the adhesive polymerizesand therefore secures the metal fairing to the structure of the airfoil.In addition, the second primary layer expands and exerts pressure on themetal fairing so as to optimize bonding.

Similarly, the first primary layer also expands under the effect of theheating and contributes to the pressure being exerted by the device.

Furthermore, the second primary layer may be of thickness lying in therange 1 millimeter (mm) to 2 mm, and is preferably 1.5 mm thick.Surprisingly, induction heating is maximized by such a thickness.

In a variant, the device of the invention includes a secondary layer ofthermo-expandable material disposed between said rigid yoke and saidfirst primary layer. Thereafter, the bonding device is provided insuccession with a non-deformable rigid yoke, with the secondary layer,with the first primary layer, with the induction heater means, and thenwith the second primary layer.

The second primary layer serves to insulate the yoke of the device sothat the yoke is not heated by the induction heater means, the heatermeans devoting its energy essentially to heating the metal fairing.

Thus, the secondary layer advantageously presents a thickness lying inthe range 6 mm to 8 mm.

Optionally, a metal plate is arranged between said secondary layer andsaid first primary layer.

The metal plate is heated by the induction heater means because of thedistribution of the magnetic field lines generated by the heater means.It can thus heat the first primary layer and the secondary layer so thatthey expand and contribute to causing the bonding device to applypressure to the metal fairing.

In order to maximize heating of the metal plate, the first primary layerhas a thickness lying in the range 1 mm to 2 mm, and is preferably 1.5mm thick.

The pressure generated by the bonding device contributes effectively tobonding the metal fairing to the airfoil. Nevertheless, surprisingly,this pressure can become harmful above a certain threshold.

If the pressure is too high, then the adhesive runs the risk of beingexpelled from the space between the metal fairing and the airfoil.

Consequently, the device of the invention optionally includes energyabsorber means, e.g. rated springs, that are secured to said yoke.

Depending on the variant used, the absorber means should be arrangedbetween the rigid yoke and the first primary layer, or else between therigid yoke and the secondary layer.

In addition, it is advantageous to associate the bonding device withmonitoring means for monitoring the pressure exerted by the device onthe metal fairing in order to ensure that the pressure remains withinacceptable limits.

The monitoring means are thus provided with an optical fiber embedded inthe second primary layer, for example, and connected to a monitoringmember.

Finally, it is possible to optimize the shape of the induction heatermeans to avoid creating hot zones that would lead to non-uniform heatingof the metal fairing.

The induction heater means then comprises four groups of Litz wires,first and second groups being disposed between the first and secondprimary layers so as to be situated on the suction side of the fairing,and third and fourth groups being disposed between the first and secondprimary layers so as to be situated on the pressure side of the metalfairing.

Furthermore, the first group is separated from the second group so as torelease a non-heated space, the third group also being separated fromthe fourth group.

Thus, the magnetic field lines generated by the heater means heat themetal fairing entirely uniformly.

The present invention also provides a bonding method enabling rapid andnon-damaging bonding to be performed between a metal fairing and anairfoil.

According to the invention, a method of bonding a protective metalfairing on the leading edge of an aircraft airfoil, with adhesive beingdisposed between the metal fairing and the airfoil, and with the bondingdevice being arranged around an outside face of the metal fairing, isremarkable in that the following steps are performed in succession:

a) induction heating the metal fairing with the help of the bondingdevice to reach a first temperature at which the adhesive is liquefiedso as to wet the airfoil and the metal fairing;

b) at the end of a first duration, increasing the temperature of thefairing to reach a second temperature enabling the adhesive topolymerize;

c) at the end of a second duration, increasing the temperature of themetal fairing to reach a third temperature in order to enablepolymerization to be complete and uniform; and

d) at the end of a third duration, causing the temperature of said metalfairing to decrease.

The method is particularly effective when it is implemented by thedevice of the invention when fitted with induction heater means enablingthe fairing to be raised very quickly to temperature.

During step a), the adhesive thoroughly wets the metal fairing and thestructure of the airfoil, thereby guaranteeing uniform bonding.

During step b), polymerization of the adhesive starts so that thebonding becomes effective.

Finally, during step c) the temperature is raised to a relatively hightemperature, since the third temperature is higher than the theoreticalpolymerization temperature of the adhesive, but for a short durationonly, thereby terminating polymerization of the adhesive.

To optimize the duration of the method, the first duration lies in therange 10 minutes (min) to 20 min and the third duration lies in therange 3 min to 10 min.

Similarly, the second duration, lying in the range 10 min to 20 mincomes to an end when the polymerization of the adhesive reaches 60% to80% of complete polymerization of the adhesive.

It should be observed that the third duration is very short so as tooptimize the total duration of the method that is implemented. In orderto achieve such a third duration, the third temperature isadvantageously higher than the polymerization temperature of theadhesive. It then becomes possible to finish off polymerization of theadhesive and thus obtain total conversion of the adhesive from theliquid state to a solid state within a short length of time.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its advantages appear in greater detail in the contextof the following description of embodiments given by way of illustrationand with reference to the accompanying figures, in which:

FIG. 1 is a section through a variant of the invention;

FIG. 2 is a diagrammatic view showing absorber means;

FIG. 3 is a section showing an optimized arrangement of the heatermeans; and

FIG. 4 is a graph explaining the steps of the invention.

Elements that are present in two or more figures are given the samereference in each of them.

MORE DETAILED DESCRIPTION

FIG. 1 is a section view showing a variant of the invention.

An adhesive 9 is placed at the end of an airfoil 7, a rotorcraft blade,for example, in order to secure a metal fairing 6 on the leading edge atthis end of the airfoil.

Furthermore, a bonding device is arranged on the outside face F1 of themetal fairing to enable the adhesive to polymerize, where polymerizationis a chemical process whereby an adhesive bonds to materials by beingheated.

The bonding device 1 comprises a rigid yoke having arranged thereininduction heater means 30 for heating the metal fairing 6.

More precisely, the induction heater means are inserted between firstand second primary layers 5′ and 5″ of a thermo-expandable material suchas silicone.

In addition, in a variant of the invention, the bonding device 1 isprovided with a secondary layer 3 made of an insulatingthermo-expandable material and of a metal plate 4, e.g. made oftitanium.

The bonding device 1 then comprises in succession: a rigid yoke 2; asecondary layer; a metal plate 4; a first primary layer 5′; inductionheater means 30; and a second primary layer 5″ that fits closely to theoutside surface F1 of the metal fairing 6.

Thus, when control means (not shown in the figures) are used to operatethe induction heater means, the metal fairing is heated and in turnheats the adhesive to cause it to polymerize.

Furthermore, the induction heater means 30 also heat the metal plate 4.

Under such conditions, the second primary layer 5″ in contact with themetal fairing 6 is subjected to an increase in temperature and begins toexpand because of its material being thermo-expandable. The same appliesto the secondary layer 3 and the first primary layer 5′ that are incontact with the metal plate 4.

Since the yoke 2 is sufficiently rigid not to be deformable, theconsequence of the thermo-expandable layers 5′, 5″, and 3 expanding isto create a pressure force from the bonding device 1 against the metalfairing 6, with this pressure force encouraging adhesion between themetal fairing 6 and the end of the airfoil 7.

In order to maximize the adhesion process, the first and second layers5′ and 5″ are of thickness lying in the range 1 mm to 2 mm, preferablyequal to 1.5 mm. Thus, the induction heater means 30 is at an optimumdistance from the elements that it is to heat in uniform manner, i.e.the metal fairing 6 and the metal plate 4.

Tn contrast, the secondary layer 3 is of a thickness lying in the range6 mm to 8 mm. This secondary layer thus insulates the yoke 2 so that itis not heated by the induction heater means.

Furthermore, the induction heater means are advantageously provided witha coil comprising a single wire, commonly referred to by the personskilled in the art as a “Litz wire”. Such a Litz wire is made up ofmutually insulated individual strands that are distributed sinusoidallywithin external protection, each strand periodically occupying all ofthe possible locations inside the external protection. Statistically,all of the strands are thus subjected to the same electromagneticconstraints. As a result, all of the strands convey similar currents,which implies that current density within the wire is uniform.

The induction heater means thus comprise a coil having a circuit 30′,referred to for convenience as the “go” circuit, that is arrangedbetween the first and second primary layers 5′, 5″ within the rigidyoke.

In addition, the coil of the induction heater means 30 has a circuit30″, referred for convenience as the “return” circuit. The returncircuit 30″ is arranged in a metal sheath 22 that is thermally insulatedwith the help of an insulating layer 21.

Advantageously, the metal sheath 22 surrounding the return circuit 30″of the induction heater means 30 is spaced apart from the rigid yoke 2by a distance L of not less than 100 mm so that the return circuit doesnot have any influence on the magnetic field created by the go circuit30′.

Finally, in order to control the heating cycle of the bonding device 1,the control means act firstly on the frequency, of the order of 50kilohertz (kHz), that is applied to the heater means, and secondly onthe current density that is delivered, where the current density is ofthe order of 4×10⁶ to 6×10⁶ amps per square meter (A/m²).

In addition, it can be advantageous to monitor the pressure exerted bythe bonding device 1 against the metal fairing 6 as a result of theexpansion of the first and second primary layers 5′ and 5″ and of thesecondary layer 3. Excessive pressure can have harmful consequences,since the adhesive can then be expelled from the airfoil by the adhesivecreeping when subjected to high pressure.

Consequently, the bonding device is optionally provided with pressuremonitoring means (not shown). Pressure monitoring can be obtained bymeans of an optical fiber connected to a monitoring member, the memberbeing incorporated in the control means of the bonding device 1, forexample.

The optical fiber is then disposed in one of the layers of the bondingdevice 1, preferably in the second primary layer 5″.

A variation in the pressure exerted on the metal fairing gives rise to avariation in the pressure exerted on the optical fiber, and thus to theintensity of the light it conveys. As a function of this lightintensity, the control member deduces the pressure being exerted andcommunicates this information to the control means that, for example,can then switch off the induction heater means in the event of thepressure being excessive.

Similarly, with reference to FIG. 2, it is possible to provide energyabsorber means that are secured to the rigid yoke 2 for absorbing energyin the event of the thermo-expandable materials expanding excessively.

The energy absorber means then comprise rated springs 8 that are securedto the rigid yoke 2 and also to the second secondary layer 3. If thepressure exerted by the bonding device on the metal fairing 6 tends toexceed a predetermined threshold, then the rated springs 8 retract,thereby enabling the pressure exerted on the metal fairing to bemaintained at no more than the level of said threshold.

It should be observed that if the variant of the invention that is beingimplemented does not include a secondary layer 3 or a metal plate 4,then the rated springs should be secured to the second primary layer 5′.

FIG. 3 is a section showing an optimized arrangement for the heatermeans.

The go circuit 30′ of the induction heater means 30 arranged between thefirst and second primary layers then have four separate groups of Litzwires, with an insulating material such as silicone being disposedbetween adjacent pairs of groups.

Subsequently, first and second groups 11, 12 are disposed on the suctionside of the metal fairing 6. Similarly, third and fourth groups 13, 14are arranged on the pressure side of the metal fairing.

This particular arrangement of the induction heater means serves toavoid forming hot points that would result in non-uniform heating beingimplemented by the bonding device 1.

In addition, in order to be able to adapt the device to any airfoil, itis possible to envisage making it as two distinct blocks that areconnected together by springs at the leading edge of the metal fairing.

The first block would then comprise the first and second groups 11 and12 and the second block would then possess the third and fourth groups13 and 14.

FIG. 4 shows the method implemented by the bonding device of theinvention.

In a preliminary step, an operator begins by placing an adhesive on theend of the airfoil 7 that is to receive the metal fairing on its leadingedge.

Thereafter, the operator puts the metal fairing in place and thenarranges the bonding device 1 on the metal fairing.

Bonding proper can then begin.

During a step a), the control means activate the induction heater meansso that the metal fairing reaches a first temperature t1. Temperaturethen rises quickly, at a rate lying in the range 2° C./min to 10°C./min.

If it is assumed that the adhesive is an adhesive of the 120° C. class,i.e. an adhesive that polymerizes at 120° C., then 6 min are required toreach a first temperature t1 of about 80° C. starting from an ambienttemperature t0 of 20° C.

At this temperature t1, the adhesive is sufficiently fluid to completelywet the surfaces with which it is in contact, i.e. the surface of theend of the airfoil and the surface of the metal fairing, as is essentialfor obtaining uniform bonding. Said surfaces are fully wetted when theinterface zone between the surfaces for bonding together is free fromany bubbles of air, such that the interface zone includes only theadhesive distributed in uniform manner so as to impregnate the fabricsfor bonding together.

At the end of a first duration d1 lying in the range 10 min to 20 min,step b) begins.

The control means then raises the temperature of the metal fairing 6 ata rate lying in the range 5° C./min to 10° C./min so as to reach asecond temperature t2 that enables polymerization to begin.

Since the adhesive is of the 120° C. class, the second temperature atwhich polymerization starts is about 110° C., with this secondtemperature depending on the chemical constituting the adhesive beingused.

At the end of a second duration d2, lying in the range 10 min to 20 min,polymerization of the adhesive reaches 60% to 80% of completepolymerization.

To reduce the heating time, at the end of this second duration d2, thecontrol means engages a step c) for terminating polymerization. It thenincreases the temperature of the metal fairing at a rate lying in therange 5° C./min to 15°/min, but possibly as great as 30° C./min shouldthat be necessary, in order to reach a third temperature t3 of about140° C. when the adhesive is of the 120° C. class.

The control means then maintains this third temperature for a thirdduration d3 that is very short, lying in the range 3 min to 10 min,prior to causing the temperature to decrease progressively, at a rate ofthe order of 2° C./min during a step d).

The bonding method is consequently very fast. In addition, because ofthe specific nature of the heater means, heating is performed uniformlyover the entire area of the metal fairing.

Furthermore, high temperatures t3 are maintained for a duration d3 thatis short and therefore does not risk damaging the airfoil.

Naturally, the present invention is capable of being subjected tonumerous variations as to its implementation. Although severalembodiments are described, it will readily be understood that it is notconceivable to identify exhaustively all possible embodiments. It isnaturally possible to envisage replacing any of the means described byequivalent means without going beyond the ambit of the presentinvention.

In particular, the values of the first, second, and third temperatures,and also the values of the above-mentioned first, second, and thirddurations depend on the adhesive that is used, and they apply morespecifically to an adhesive of the 120° C. class.

Depending on the characteristics of the adhesive that is used forbonding the leading-edge fairing, these various values could naturallybe different from those mentioned, without thereby going beyond theambit of the invention, the method itself remaining identical.

What is claimed is:
 1. A bonding device for bonding a metal fairing forprotecting a leading edge of an aircraft airfoil, said metal fairinghaving a shape and being for bonding against said airfoil with the helpof an adhesive, said device comprising a rigid yoke in which inductionheater means are inserted for heating said metal fairing, and whereinsaid induction heater means are arranged between first and secondprimary layers of a thermo-expandable material, said second primarylayer matching to the shape of said metal fairing while said device isin use, wherein said induction heater means comprises four groups ofLitz wires, first and second groups being disposed between said firstand second primary layers so as to be situated on a suction side of saidfairing, and third and fourth groups being disposed between said firstand second primary layers so as to be situated on a pressure side ofsaid metal fairing, and wherein said first group is separated from saidsecond group so as to release a non-heated space, said third group alsobeing separated from said fourth group.
 2. The device according to claim1, wherein said first primary layer has a thickness lying in the range 1mm to 2 mm and said second primary layer has a thickness lying in therange 1 mm to 2 mm.
 3. The device according to claim 1, including asecondary layer of thermo-expandable material disposed between saidrigid yoke and said first primary layer.
 4. The device according toclaim 3, wherein said secondary layer has a thickness lying in the range6 mm to 8 mm.
 5. The device according to claim 3, wherein a metal plateis arranged between said secondary layer and said first primary layer.6. The device according to claim 1, including energy absorber meanssecured to said yoke.
 7. The device according to claim 1, the devicebeing provided with monitoring means for monitoring the pressure exertedby said device on said fairing.
 8. The device according to claim 7,wherein said monitoring means are provided with an optical fiberconnected to a monitor member, wherein said optical fiber is embedded insaid second primary layer.
 9. The device according to claim 1, whereinsaid second primary layer has a curved shape.
 10. The device accordingto claim 1, wherein said induction heater means has a curved shape andsaid first and second primary layers have a curved shape.
 11. The deviceaccording to claim 1, wherein said thermo-expandable material comprisessilicone.
 12. The device according to claim 1, wherein said first andsecond primary layers of thermo-expandable material are capable ofexpanding to exert pressure on said metal fairing exposed to heat fromsaid induction heater means.
 13. A bonding device for bonding a metalfairing to an aircraft airfoil for protecting a leading edge of theaircraft airfoil, said metal fairing having a curved shape and being forbonding against said airfoil with the help of an adhesive, said devicecomprising a rigid yoke housing an induction heater for heating saidmetal fairing, said induction heater being arranged between first andsecond primary layers of a thermo-expandable material, said secondprimary layer being of a shape to correspond to the curved shape of saidmetal fairing and being housed within and adjacent the rigid yoke,wherein said induction heater comprises four groups of Litz wires, firstand second groups being disposed between said first and second primarylayers so as to be situated on a suction side of said fairing, and thirdand fourth groups being disposed between said first and second primarylayers so as to be situated on a pressure side of said metal fairing,and wherein said first group is separated from said second group so asto release a non-heated space, said third group also being separatedfrom said fourth group.
 14. The device according to claim 13, whereinsaid induction heater has a curved shape and said first and secondprimary layers have a curved shape, and wherein said thermo-expandablematerial comprises silicone.
 15. A bonding device for adhesively bondinga metal fairing having a shape to an aircraft airfoil, said devicecomprising: a non-deformable rigid yoke having a first curved surface; afirst primary layer of thermo-expandable material secured adjacent tosaid yoke, said first primary layer having a second curved surfacedisposed within and facing said first curved surface; an inductionheater for heating said metal fairing, said heater secured within tosaid first primary layer of thermo-expandable material; and a secondprimary layer of a thermo-expandable material secured adjacent to saidinduction heater, wherein said second primary layer corresponds to theshape of said metal fairing while said device is in use, said inductionheater is disposed between said first and second primary layers, whereinsaid induction heater comprises four groups of Litz wires, first andsecond groups being disposed between said first and second primarylayers so as to be situated on a suction side of said fairing, and thirdand fourth groups being disposed between said first and second primarylayers so as to be situated on a pressure side of said metal fairing,and wherein said first group is separated from said second group so asto release a non-heated space, said third group also being separatedfrom said fourth group.
 16. The device according to claim 15, whereinsaid induction heater has a curved shape.
 17. The device according toclaim 15, wherein said thermo-expandable material comprises silicone andwherein said first and second primary layers of thermo-expandablematerial are capable of expanding to exert pressure on said metalfairing exposed to heat from said induction heater means.